1. Field of the Invention
The present invention relates to a magnetoresistance type sensor device for detecting a change of an angle or strength of an applied magnetic field in terms of an electric signal by using magnetoresistance elements interconnected in the form of a Wheatstone bridge circuit. More particularly, the present invention is concerned with improvement of the magnetoresistance type sensor device such that the output signal thereof is made to be substantially immune to the influence of variations in the ambient temperature by compensating for temperature characteristics of the magnetoresistance elements. The magnetoresistance type sensor device according to the invention can find profitable applications for detection of an angular position or displacement (stroke) of a member such as, for example, a steering column of a motor vehicle and others with high accuracy and reliability. Parenthetically, it should first be mentioned that with the phrase "magnetoresistance element", it is intended to mean circuit elements whose electric resistance or whose voltage generated thereacross changes in dependence on an angle at which a magnetic field is applied or strength thereof and encompass a ferromagnetic thin-film element made of a magnetic material having considerably high permeability such as Ni-Co., a Hall element and the like.
2. Description of Related Art
The magnetoresistance type sensor devices mentioned above are well known in the art, and they are generally so designed as to generate a signal indicative of an angular position and/or displacement of a member being monitored with the aid of magnetoresistance elements which are interconnected in the form of a Wheatstone bridge circuit and whose resistances vary in response to change in an angle or strength of a magnetic field applied externally from a permanent magnet or the like element provided in association with the member of concern.
Before entering into description of the present invention, background techniques will be explained in some detail for having a better understanding of the invention.
FIG. 3 is a circuit diagram showing a conventional magnetoresistance type sensor device known heretofore and employed generally for detecting an angular position, a stroke or displacement of a member of concern, and FIG. 4 is a view for illustrating disposition and patterns of magnetoresistance elements employed in the magnetoresistance type sensor device shown in FIG. 3 together with a magnetic field applied to the device.
Referring to FIG. 3, the magnetoresistance type sensor device includes a Wheatstone bridge circuit 1 which is comprised of first to fourth magnetoresistance elements RA, RB, RC and RD. As the magnetoresistance elements RA, RB, RC and RD, there may be used a ferromagnetic thin-film element made of a magnetic material having considerably high permeability such as Ni-Fe, Ni-Co, etc., a semiconductor magnetoresistance element, a Hall element and the like, as mentioned previously. In the following description, it is however assumed that the ferromagnetic thin-film magnetoresistance element is employed. It should further be mentioned that a constant current circuit is provided for supplying a constant current to the Wheatstone bridge circuit 1, although it is not shown in FIG. 3.
The magnetoresistance elements RA and RB are interconnected at a junction P1 (neutral point), the magnetoresistance elements RC and RD are connected together at a junction P2 (neutral point) and the magnetoresistance elements RA and RC are interconnected at a junction P3 with the magnetoresistance elements RB and RD being interconnected at a junction P4, wherein the junctions P1 and P2 constitute output terminals of the Wheatstone bridge circuit 1, while the junction P3 is connected to a power source V.sub.cc with the junction P4 being connected to the ground potential. In FIG. 3, reference character V1 represents a potential at the junction P1 with V2 representing a potential at the junction P2.
A differential amplifier circuit 2 is connected to the junctions P1 and P2 constituting the output terminals of the Wheatstone bridge circuit 1 and has an inverting input terminal (-) to which the potential V1 of the junction P1 is applied and a non-inverting input terminal (+) to which the potential V2 of the junction P2 is applied.
Further, the non-inverting input terminal (+) of the differential amplifier circuit 2 is connected to a junction between variable voltage divider resistors R1 and R2 which are connected in series and inserted between the ground potential and the power source V.sub.cc so that a reference voltage for the potential V2 is applied to the non-inverting input terminal (+) of the differential amplifier circuit 2. Thus, there is finally obtained a differential amplitude voltage signal V0 from the magnetoresistance type sensor device, which signal indicates changes in an angle at which an external magnetic field is applied to the magnetoresistance type Wheatstone bridge circuit or change in the strength of the applied magnetic field.
Next reference is made to FIG. 4. In this figure, reference characters 1, RA to RD and P1 to P4 denote the parts which are equivalent to those denoted by same reference characters in FIG. 3. As can be seen in FIG. 4, the magnetoresistance elements RA to RD are each deposited in a zigzag pattern and disposed in juxtaposition to one another on a same plane of a substrate. In this conjunction, it is to be noted that the zigzag patterns of the magnetoresistance elements RA and RB connected to each other at the junction P1 (constituting the first output terminal of the Wheatstone bridge circuit 1) extend in the directions opposite to each other (i.e., symmetrically relative to a vertical center line extending through the junctions P1 and P2 of the Wheatstone bridge circuit 1, as viewed in FIG. 4). The zigzag patterns of the magnetoresistance elements RC and RD which are connected together at the junction P2 constituting the second output terminal of the Wheatstone bridge circuit 1 are also disposed similarly to the magnetoresistance elements RA and RD.
On the other hand, the magnetoresistance elements RA and RD disposed diagonally opposite to each other are deposited in the same zigzag pattern. Similarly, the magnetoresistance elements RB and RC are deposited in a same zigzag pattern which extend in the directions orthogonally to those of the magnetoresistance elements RA and RD, respectively.
By virtue of the patterning and disposition of the magnetoresistance elements RA to RD described above, there make appearance at the junctions P1 and P2 the potentials V1 and V2 of the polarities opposite to each other, whereby a differential amplitude voltage signal (i.e., a voltage signal indicative of a difference between the potentials V1 and V2) is outputted from the Wheatstone bridge circuit 1.
Now, let's assume that an external magnetic field H is applied to the Wheatstone bridge circuit 1 at an angle .theta., as illustrated in FIG. 4. Such magnetic field H may be generated by a permanent magnet of a sensor mounted on a rotatable member whose angle is to be detected, e.g. a steering column of a motor vehicle and applied to the Wheatstone bridge circuit 1 mounted stationarily in a predetermined orientation.
On the above assumption, operation of the conventional magnetoresistance type sensor device will now be elucidated by reference to FIGS. 3 and 4.
When the magnetic field H is applied to the magnetoresistance elements RA to RD of the Wheatstone bridge circuit 1, the resistance values of the magnetoresistance elements RA to RD will change in dependence on the change in the angle .theta. of the applied magnetic field.
Accordingly, the potentials V1 and V2 appearing at the junctions P1 and P2, respectively, will change in dependence on the change of the angle .theta. of the applied magnetic field. The potentials V1 and V2 are applied to the plus and minus input terminals of the differential amplifier circuit 2 to be thereby differentially amplified, which ultimately results in generation of the amplitude voltage signal V0 from the differential amplifier circuit 2.
On the basis of the differential amplitude voltage signal V0 generated in this way, it is possible to detect the angle of rotation, angular stroke or displacement or the like of a member being monitored (e.g. a steering column of a motor vehicle) in association with which the magnetoresistance type sensor device is provided.
In this conjunction, it is noted that the temperature coefficients of resistance of the magnetoresistance elements RA to RD which assume different resistance values in dependence on the angle .theta. of the applied magnetic field exert significant influence to the characteristic of the differential amplitude voltage signal V0 which is derived from the potentials V1 and V2 at the junctions P1 and P2 of the Wheatstone bridge circuit 1. This will be analyzed below.
In general, the resistance value R of each of the magnetoresistance elements RA to RD can be given by the following expression (1): EQU R=R.sub.min {1+.alpha..sub.min (T-25)}sin.sup.2 (.pi./4-.theta.) +R.sub.max {1+.alpha..sub.max (T-25)}cos.sup.2 (.pi./4-.theta.) (1)
where R.sub.min represents a minimum resistance value which the magnetoresistance elements RA to RD can assume, R.sub.max represents a maximum resistance value which the magnetoresistance elements RA to RD can assume, .alpha..sub.min represents a minimum temperature coefficients of resistance of each of the magnetoresistance elements RA to RD, and .alpha..sub.max represents a maximum temperature coefficients of resistance of each of the magnetoresistance elements RA to RD, and T represents an ambient temperature on the assumption that the normal or room temperature is 25.degree. C.
Now, let's suppose that the angle .theta. of the magnetic field applied to the magnetoresistance elements RA to RD changes within a range covering (-).pi./4 to (+).pi./4. Then, the terms "sin.sup.2 (.pi./4-.theta.)" and "cos.sup.2 (.pi./4-.theta.)" in the above expression (1) assume "1" or "0" for the maximum value and the minimum value of the angle .theta. of the magnetic field. In this case, the amplitude voltage signals Vs1 and Vs2 derived from the potentials V1 and V2 at the junctions P1 and P2, respectively, can be given by the undermentioned expressions (2) and (3), respectively. ##EQU1##
Now, the temperature characteristics of the amplitude voltage signals Vs1 and Vs2 will be considered. Magnitudes of change .DELTA.Vs1 and .DELTA.Vs2 of the amplitude voltage signals Vs1 and Vs2 for a temperature change .DELTA.T (=T-25) from the room temperature (25.degree. C.) to the ambient temperature T can be given by the following expressions (4) and (5), respectively. ##EQU2##
Thus, the differential output signal (.DELTA.Vs1-.DELTA.Vs2) between the voltage changes .DELTA.Vs1 and .DELTA.Vs2 of the output voltage signals Vs1 and Vs2 of the Wheatstone bridge circuit 1 can be expressed as follows: ##EQU3##
As can be seen from the above expression (6), the differential output signal (.DELTA.Vs1-Vs2) which corresponds to the differential amplitude voltage signal V0 of the differential amplifier circuit 2 is subjected to the influence of the nonlinear temperature characteristics of the amplitude voltage signals Vs1 and Vs2 given by quadratic expressions of the minimum resistance value R.sub.min and the maximum resistance value R.sub.max as a function of change of the ambient temperature.
Such being the circumstances, the differential amplitude voltage signal V0 outputted ultimately from the magnetoresistance type sensor device is subjected to the influence of the temperature characteristics of the individual magnetoresistance elements, which makes it very difficult to effectuate the temperature compensation over the whole range of temperatures to which the Wheatstone bridge circuit 1 is exposed, even when the constant current circuit power source (not shown) is employed, as mentioned previously.
From the above description, it is apparent that the magnetoresistance type sensor device known heretofore suffers from a problem that the temperature compensation is very difficult to effectuate over the whole range of temperatures to which the resistance elements RA to RD are exposed, even when the constant current power source circuit is employed, because of the non-linear temperature characteristics expressed as a quadratic equation of the minimum resistance value R.sub.min and the maximum resistance value R.sub.max of the magnetoresistance elements RA to RD, respectively, as a function of the ambient temperature T due to the zigzag patterning of the magnetoresistance elements RA to RD symmetrical relative to the center point of the Wheatstone bridge circuit, as shown in FIG. 4.